1. Field of the Invention
This invention relates generally to methods and apparatus for supplying electrical power to ozone generators, and more specifically to a method for adjusting the peak amplitudes of discontinuous waveforms for powering corona discharge cells.
2. Description of the Prior Art
It is extremely desirable to be able to supply ozone generating cells with a high-voltage (KV) waveform in which the negative and positive peak voltages are identical or nearly so, because the most efficient ozone generation occurs over a relatively narrow voltage range. If the negative and positive KV peaks are unequal, then when one peak is at the optimum KV, the other will be either above or below the optimum. The higher the voltage, the greater is the potential for corona hot-spots which reduce ozone generation efficiency, particularly in the case of non-uniformities of the corona gap where the gap is narrower. In addition, as the voltage increases, dielectric breakdown and failure of transformer insulation become more likely both between secondary turns and from secondary to primary, and dielectric breakdown and parasitic arcing begin to occur within the corona cell. At higher voltages, more corona forms on the outer surfaces of the high-voltage transformer, high-voltage leads and the corona cell, particularly if the ambient air is moist and/or the surfaces are not clean. This external corona produces unwanted ozone outside of the corona cell which degrades these surfaces and can escape into the ambient air.
Thus, it is desirable to drive the corona at the lowest peak KV possible. On the other hand, if the peak KV is too low, the corona will not ignite evenly in all areas, especially if there are non-uniformities where the gap is wider. The range of optimum peak KV for the most efficient ozone generation begins somewhat above the minimum KV necessary to produce a stable corona, and ends below where corona hot spots begin to occur. This optimum range becomes narrower and more critical as the corona gap becomes less uniform.
The use of a discontinuous bipolar pulse waveform (e.g., the single-cycle bipolar pulse described in the copending patent application) for powering corona discharge in ozone generators has many advantages. However, in some systems this waveform introduces a new problem which does not occur with conventional continuous-wave waveforms: an inequality of the peak KVs. When supplying the primary of the transformer with a symmetrical voltage bipolar pulse squarewave, the overall maximum level of KV output from the secondary of the transformer to the ozone generating cell(s) can be adjusted by varying the amplitude and/or pulse width of the primary bipolar squarewave. The actual KV peaks of the output will not always be equal in amplitude, e.g., the positive KV peak will often be higher than the negative KV peak (except for particular combinations of transformer inductance and capacitance, corona cell capacitance, and bipolar pulse width). Normally the second KV peak will be higher than the first, because it has the advantage of being the second swing of the "pendulum". In other words, part of the energy of the first pulse goes into charging up the LCR (inductance, capacitance and resistance) circuit, and then when the polarity swings into the second pulse some of this charge carries over, allowing the second pulse to swing higher than the first. This energy is then dissipated by the corona current and to a much smaller extent by damped oscillations of the capacitative current. After a dead time interval between bipolar pulses, the next first pulse has to start from scratch again. Furthermore, the inductance, capacitances, impedance, reactance and pulse widths often interact to create resonances which result in a skewing of the waveform and its peak voltages. The difference between the peak voltages can change when any of the above variables change. Attempting to adjust the ratio of the KV peaks (by making the amplitudes or widths of the unipolar pulses of the bipolar pulse which supply the transformer different) introduces complexity into the power supply, especially if a continuous type of adjustment is used. In addition, introducing a difference in unipolar pulse amplitudes or widths usually creates more skewing than it can cure.